Burn the boats or dip your toe?
In an apocryphal story the Vikings burn their boats on arrival at the New World to force them to focus all in. Are you the type of person to act like this? Or would you feel more comfortable with a gradual exposure, keeping the option of the regular work you know and can return to?

I think this is very personal and situational. I own part of a medical practice while writing this and so I want niche work to feed towards that practice. I therefore have the assurance of general work always there in the practice, which may slowly ratchet down as niche work ratchets up.
However if I was footloose and fancy free, perhaps I would prefer to set up my own shop for total control. Going all in means you can really devote all your time and energies to this one project – obviously if you other sources of funds to keep you going like savings, partner or a part-time job elsewhere that allows you to focus purely on developing this new niche.
Nope, I’m out of here – setting up on your own
Maybe you just really want a break and a chance to set up on your own. This can be a way to ensure you dedicate your efforts to the one thing – this new niche. This ensures you have very clear differentiation from a seeing-all-comers GP clinic and get known for just this work. You will not have a regular patient trying to squeeze their husband in for a BP check or just the drivers licence form during the niche work you need to focus on. Setting up on your own allows very specific clear branding, marketing and price setting which is only around your niche.
Having a clean break and setting up by yourself also ensures it is very clear that you own all the intellectual property, databases and patient list. This could be less clear working from an existing clinic. This is important if we think long term about selling your niche or business to retire or move on to the next thing. This is why looking at your service contract is important. In older service contracts that were probably more employment models, there could be clauses about the practice retaining intellectual property and geographical restraint of practice if you left. However with the changes to Payroll tax and the improved focus on tenant doctor models being finessed with better agreements, your tenant doctor agreement should not discuss IP or geographical restriction, for fear it turns you into a deemed employee and therefore makes the practice liable for payroll tax.
Setting up on your own may of course have increased costs compared to starting where you are. You will need your own technology, software stack, and place to work. You may need to employ people (administration, nursing). You will need a way to get paid and banking and reconciliation software to allow business banking and tax reporting. You may need to buy specific equipment, some so expensive that you need chattel loans to support you. These costs relate to some of what I discussed in the training/costs section – are there some calculations you can devise that assess what revenue you need from patients to make this all worth while? If you are going to need a loan, the bank will want to see a business plan that surveys the current area/market, expected revenue, opportunities for growth and assesses risks.